The present invention relates to electronic ballasts for powering gas discharge lamps and particularly to DC/AC inverters for energizing and dimming gas discharge lamps, including inductively coupled gas discharge lamps.
A gas discharge lamp typically utilizes an electronic ballast for converting AC line voltage to high frequency current for powering the lamp. Conventional electronic ballasts include an AC to DC converter and a resonant inverter converting DC voltage to lamp high frequency current. The resonant inverter includes switching transistors generating a high frequency rectangular AC voltage that is applied to a voltage resonant circuit having an inductor and a capacitor in series. The gas discharge lamp is coupled in parallel to the capacitor. For high frequency electronic ballasts, a self-oscillating resonant inverter is a common part that generates AC voltage for starting and AC current for powering the lamp. Self-oscillating resonant inverters utilize a feedback transformer coupled between a resonant circuit and gates of the switching transistors to provide a sinusoidal voltage to the gates for sustaining the oscillations. Dead time intervals for the switching transistors are automatically formed when crossing near zero gate voltage providing zero voltage switching (ZVS). Self-oscillating resonant inverters are described, for example, in U.S. Pat. Nos. 4,748,383, 5,962,987 and 5,982,108.
A typical voltage feedback self-oscillating circuit is shown in FIG. 1. The self-oscillating inverter is self-adjusted to above resonant frequency. If resonant frequency changes with temperature, lamp inductance and load variations, the inverter will still operate above the resonant frequency. However, driving MOSFET's with a sinusoidal voltage causes dead time variations followed by lamp power variations and other disadvantages.
As shown in FIG. 1, a prior art electronic ballast for powering a gas discharge lamp converts a standard AC line voltage to a high frequency current for lighting the lamp. An AC/DC converter is coupled to the AC line through an EMI filter. The AC/DC converter includes a rectifier (not shown) and, optionally, a power factor corrector. An AC/DC converter output low frequency ripple is filtered out by an electrolytic storage capacitor C31 connected across a high voltage DC bus. The self-generating ballast inverter is connected to the DC bus and its output is connected to the lamp. The storage capacitor C31 reduces the high frequency voltage ripple across the DC bus. Two series switching MOSFET's M1 and M2 are coupled across the DC bus. A resonant load includes series an inductor L1, a capacitor C3, and the lamp that is coupled in parallel to the capacitor C3. The resonant load is connected in parallel to the switching MOSFET M2 via a DC blocking capacitor C1. The switching transistors M1 and M2 are driven through resistors R16 and R15 via voltage feedback circuit that includes transformer T9 and an output voltage divider with capacitors C27 and C30. The ballast inverter in FIG. 1 includes a starting circuit built with a starting capacitor C29, a resistor R19 charging the starting capacitor C29 from the DC bus, a discharged iac X28 coupled to the starting capacitor C29 and blocking diode D10 for discharging the starting capacitor C29 after inverter starting.
With the feedback circuit in FIG. 1, the switching phase of the inverter transistors is locked with the phase of the output voltage oscillations load, thereby preventing the inverter from running below resonant frequency.
Ballasts with high frequency integrated circuit (IC) oscillating inverter controllers, such as the IR 215X series from International Rectifier or the L6579 series from ST Microelectronics, do not have the drawbacks of self-oscillating circuits. With shutdown and restarting features, these IC driven inverters can be used for ON/OFF pulse width modulation (PWM) dimming. However, with a preadjusted switching frequency that is not sensitive to transient variations and fluctuations of resonant frequency of the resonant load, direct application the above controllers has been troublesome. Without correction of switching frequency, MOSFET's could have cross conduction and fail when operating below resonant frequency in some steady-state conditions, dimming mode or at lamp starting.
A solution to this problem is described in Application Notes AN 995A “Electronic Ballasts Using the Cost-Saving IR215X Drivers” issued by International Rectifier. FIG. 2 illustrates a feedback circuit with two anti-parallel power diodes connected in series with the resonant load as zero current detectors. The diodes generate a rectangular AC pulse signal that forces the timing circuit for the IC to switch synchronously with this signal. A feedback signal indicates phasing of current in the resonant load. However, zero current sensing in any portion of the resonant load does not provide the necessary synchronization angle for optimized operation mode above the resonant frequency. In addition, when used as a source of synchronization signals, the power diodes add significant power losses to the ballast.
The prior art electronic ballast shown in FIG. 2 includes a self-oscillating controller for driving switching transistors M1 and M2 of the resonant inverter. A resonant load includes an inductor L, capacitor C2 and a fluorescent lamp coupled in parallel with capacitor C2. The resonant load is connected via DC blocking capacitor C1 in parallel to transistor M2. An IR2155 controller includes a timer (known as a “555” timer), with timing capacitor CT and timing resistor RT being external components that determine the oscillating frequency. Anti-parallel diodes D46 and D51 connected in series circuit with the resonant inductor L are used as zero current detectors generating a rectangular pulse signal. This signal is injected between the timing capacitor CT and ground.
Other prior art IC driven ballasts are disclosed in U.S. Pat. Nos. 5,723,953 and 5,719,472. Both patents teach IC feedback control with a resistor, placed in a manner similar to the anti-parallel diodes D46 and D51 in FIG. 2. The resistor signal indicates both the level of current in the resonant load and the phasing of this current. According to these two patents, the feedback signal is injected into the IC timing circuit during lamp starting. As a result, inverter frequency is reduced to a level of the resonant frequency and, accordingly, inverter output voltage increases for lamp starting. The higher the resistor signal, the lower the inverter switching frequency, the higher the output inverter voltage, and accordingly, the resistor signal. It was found that the prior art inverter feedback circuit may cause excessive starting voltage and operation below the resonant frequency. If it is used in a steady state mode, this positive feedback circuit creates instability.
The present inventor has found, when synchronizing prior art inverters with an open feedback loop from an independent sinusoidal control signal source and optimizing inverter switching frequency above the resonant frequency, that the signal injected into the IC timing circuit is significantly out of phase from the signals generated across the above-described current sensors. The phase difference between the inverter output voltage and the external synchronization signal is typically in a range from 150° to 200° depending on resonant load, type of IC and selected operating frequency.
It is desirable, for reliable phase lock and before closing the loop, that the injected feedback signal be generated with a minimum phase difference between the inverter output voltage and the external synchronization signal to provide an optimum mode for the loop inverter. In this case, the injected signal will dominate the ramp signal and the inverter will operate at this optimum mode after closing the loop. It is also desirable that power components, such as diodes and resistors, not be used as sensors in the resonant load to avoid additional power losses.
In any obvious possible connection of current sensing diodes (in series with resonant capacitor, inductor, or lamp), it is difficult, or even impossible to achieve an optimum mode phase lock in a resonant inverter driven by self-oscillating AC.
Therefore, there is still a need for improvements in inverter controls, in particular with more advanced controller integrated circuits.